Nuclear totipotency of cultured rabbit morulae to support full-term development following nuclear transfer.

The rabbit was used as a model for nuclear transfer. A critical step in nuclear transfer is oocyte activation, which was evaluated in this research. Optimal field strength of an electric stimulus for activation was examined. A significantly higher activation rate in all criteria tested was achieved when oocytes were activated electrically with a field strength of 2.4 kV/cm versus 1.2 or 1.8 kV/cm. Also, electrical stimulation with combined alternating current (AC) and direct current (DC) was superior to DC stimulation alone for activation. In another study involving 586 oocytes, exposure of oocytes to cytochalasin B for 1 h followed by activation with electrical stimulation significantly improved development of the oocytes to blastocyst stage compared to oocytes without cytochalasin B pre-exposure (38% vs 26%, p less than 0.05). Cytochalasin B exposure alone (control), however, had no effect on activation. Exposing oocytes to activation medium without electrical stimulation also activated some oocytes. In the nuclear transfer experiment, blastomeres from 8-cell embryos cultured for 20-24 h to the 32-64-cell stage were used as nuclear donor cells. Of 491 oocytes used, 459 (93%) survived the enucleation and fusion procedure, 370 (81%) fused, and 284 (77%) developed into 2-4-cell embryos. A total of 243 of these 2-4-cell embryos were transferred to 15 pseudopregnant recipients and produced 8 young (3%). Although the efficiency is low, this study demonstrated that rabbit morulae cultured for 20-24 h to the 32-64-cell stage as nuclear donors for transfer remain totipotent.